This invention relates to a method for producing laminated material. More particularly, the invention relates to a method for producing laminated material which is characterized in that the lamination process can be carried out quite easily, the adhesive strength between laminated layers is high and, in the lamination process, the thermal deterioration of polyolefin or its composition or the undesirable release or deterioration of active agents in the polyolefin composition are well avoided. (2) Description of the Prior Art
As a typical method for producing laminated materials, there is known an extrusion laminating method.
In this method, several kinds of substrate materials of papers such as quality paper or kraft paper, plastic films, regenerated cellulose film and metal foils are coated by molten polyethylene by extrusion lamination, thereby producing various kinds of wrapping or packaging materials and so forth utilizing the advantageous properties of the combination of a substrate material and a plastic material such as water-proofness, moisture-proofness, chemical resistance, softness, heat-sealable property, etc.
The adhesive bonding mechanisms in the above lamination process are classified into two kinds, i.e., mechanical adhesive bonding and chemical adhesive bonding. The former mechanical adhesive bonding is applied to porous substrate materials such as cloth and paper, in which the molten polymer extruded at high a temperature infiltrates into fine pores among fibers and it is then solidified by cooling providing adhesive bonding between a plastic material and a substrate material.
The latter chemical adhesive bonding utilizes the chemical intermolecular force and it is applied to the materials having smooth surfaces such as regenerated cellulose film, aluminum foil and plastic films or those having no functional groups. In order to obtain satisfactory bonding force in the chemical adhesive bonding, it is necessary that the substrate material is subjected to primer treatment, and it is often treated by corona discharge.
Meanwhile, it is also desirable that the polyethylene used for extrusion coating is activated. As the method for this activation, it is a general practice that polyethylene is extruded at higher temperatures so as to increase oxygen-containing polar groups (carbonyl groups) in molecules before it is brought into contact with a substrate material so as to provide sufficient adhesive strength.
In the former mechanical adhesive bonding, when the temperature of molten polymer is high, the viscosity of polymer is lowered to improve the infiltration into a substrate material and the adhesive strength is also improved. Therefore, the lamination is done at possibly high temperatures as far as the polymer is not decomposed. Accordingly, low density polyethylene is generally melted and kneaded by being heated to, at the lowest, 280.degree. to 310.degree. C. in the conventional lamination process. In this process, it is necessary that the polyethylene is not decomposed and does not give out any offensive odor. In the industrial practice, however, partial decomposition is caused to occur with giving off irritating smell from decomposed product, and much smoke is emitted during the processing. Thus the polyethylene suffers from serious thermal deterioration.
Meanwhile, in the case that polyolefin laminated with substrate sheet contains a drug or other agent such as insecticides and other biologically active agents, perfumes and deodorants (hereinafter referred to as "active agents"), the active agents are diffused in the polyolefin layer to reach boundary surfaces of layer, and with the passage of time, the layer sometimes peels off from the substrate sheet.
In a first method for producing a laminated material, two steps of the preparation of films to be laminated and the lamination of the prepared films are done separately. In the other second method, the polyolefin containing the active agents is laminated with a sheet material by extrusion lamination in one step process. This method is, however, defective because the thermal deterioration and undesirable release of active agents are caused to occur due to the high temperature in the extrusion lamination and, in a worse case, the thermal decomposition or thermal degradation of active agents occurs, besides the peeling of layers.
In the conventional methods, there are problems of the thermal deterioration of polyolefin and undesirable release or thermal degeneration of active agents. Even though it is intended to solve these problems by carrying out the extrusion lamination at low temperatures, it has not been successful because the adhesive strength between adjoining layers becomes very low.